1. Field of the Invention
The present invention relates to a magnetic recording medium which is employed in, for example, hard disk devices, to a process for producing the magnetic recording medium, and to a magnetic recording and reproducing apparatus. More particularly, the present invention relates to a magnetic recording medium exhibiting excellent recording and reproduction characteristics.
2. Background Art
The recording density of a hard disk device (HDD), which is a magnetic recording and reproducing apparatus, has increased by 60% per year, and this tendency is expected to continue. Therefore, magnetic recording heads and magnetic recording media which are suitable for attaining high recording density are now under development.
Magnetic recording media employed in hard disk devices are required to have high recording density, and therefore demand has arisen for improvement of coercive force and reduction of medium noise.
Most magnetic recording media employed in hard disk devices have a structure including a magnetic recording medium substrate on which a metallic film is laminated through sputtering. Aluminum substrates and glass substrates are widely employed for producing magnetic recording media. An aluminum substrate is produced through the following process: an NiP film (thickness: about 10 μm) is formed through electroless plating on an Al—Mg alloy substrate which has undergone mirror polishing, and the surface of the NiP film is subjected to mirror polishing. Glass substrates are classified into two types; i.e., amorphous glass substrate and glass ceramic substrate. When either of these two types of glass substrate is employed to produce a magnetic recording medium, the substrate is subjected to mirror polishing.
In general, a magnetic recording medium employed to produce a hard disk device includes a non-magnetic substrate; a non-magnetic undercoat layer (formed of, for example, NiAl, Cr, or a Cr alloy); a non-magnetic intermediate layer (formed of, for example, a CoCr alloy or a CoCrTa alloy); a magnetic layer (formed of, for example, a CoCrPtTa alloy or a CoCrPtB alloy); a protective film (formed of, for example, carbon), the layers and film being successively formed on the substrate; and a lubrication film containing a liquid lubricant formed on the protective film.
In order to increase recording density, signal to noise ratio (SNR) when recording is performed at high frequency must be enhanced. As described in “Magnetic Materials and Structures for Thin-Film Recording Media,” Kenneth, E. J., JOURNAL OF APPLIED PHYSICS Vol. 87, No. 9, 5365 (2000), in order to enhance SNR, the diameters of crystal grains contained in a recording layer must be reduced and made uniform.
Meanwhile, as reported in “Temperature Dependence of Thermal Stability in Longitudinal Media,” Sharat Batra et al., IEEE Trans. Magn. Vol. 35, No. 5, 2736 (1999), when the diameter of crystal grains contained in a recording layer is reduced, the volume of the crystal grains is reduced, and thus magnetization becomes thermally unstable. In order to enhance SNR of a magnetic recording medium, the diameter of crystal grains contained in a recording layer must be reduced. However, as a result, the volume of the crystal grains is reduced, and magnetization becomes thermally unstable.
Japanese Patent Application Laid-Open (kokai) No. 2001-56921 discloses a technique for solving the aforementioned problems, which employs antiferromagnetic coupling in a recording layer. This technique employs inverted magnetization of magnetic layers (i.e., recording layers) formed atop and beneath a non-magnetic coupling layer formed of, for example, ruthenium. Since the magnetization direction of the recording layer formed atop the non-magnetic coupling layer is opposite that of the recording layer formed beneath the coupling layer, a portion of each of the recording layers that participates in magnetic recording and reproduction has a thickness substantially smaller than the thickness of the recording layer. Therefore, SNR can be enhanced. Meanwhile, since the volume of crystal grains contained in the entirety of the recording layers becomes large, thermal stability of magnetization can be improved. Media employing such a technique are generally called “antiferromagnetically-coupled media (AFC media)” or “synthetic ferrimagnetic media (SFM).” In the present specification, such media will be called “AFC media.”
FIGS. 5 and 6 show the structures of conventional AFC media. The AFC medium shown in FIG. 5 has a structure in which a non-magnetic coupling layer is sandwiched by two magnetic layers. The AFC medium shown in FIG. 6 has a structure in which a non-magnetic coupling layer and a magnetic layer are laminated on the layered structure as shown in FIG. 5 including two magnetic layers. In FIG. 5, reference numeral 501 denotes a non-magnetic substrate, 502 a non-magnetic undercoat layer, 503 a non-magnetic intermediate layer, 504 a first magnetic layer, 505 a non-magnetic coupling layer, 506 a second magnetic layer, 507 a protective film, and 508 a lubrication layer. In FIG. 6, reference numeral 601 denotes a non-magnetic substrate, 602 a non-magnetic undercoat layer, 603 a non-magnetic intermediate layer, 604 a first magnetic layer, 605 a non-magnetic coupling layer, 606 a second magnetic layer, 607 a non-magnetic coupling layer, 608 a third magnetic layer, 609 a protective film, and 610 a lubrication layer.
When magnetization of the first magnetic layer is represented by “M1,” the volume of crystal grains contained in the first magnetic layer is represented by “V1,” magnetization of the second magnetic layer is represented by “M2,” and the volume of crystal grains contained in the second magnetic layer is represented by “V2,” the entire volume of the magnetic layers becomes (V1+V2). Thus, when using two magnetic layers, the entire volume of the magnetic layers increases as compared to the use of a single magnetic layer, and therefore thermal stability is enhanced. However, the entire magnetization of the magnetic layers becomes (M2−M1); i.e., the entire magnetization of the magnetic layers decreases, and thus output of the magnetic layers serving as recording layers is lowered.
As reported in “Promising SFM (Synthetic Ferrimagnetic Media) Technology,” Akira Kakeihi, Technical Conference, Session 2a, DISKCON USA2001, when the thickness of the first magnetic layer of the AFC medium shown in FIG. 5 is regulated to 5 nm, signal decay, which indicates thermal instability of the magnetic layer, is improved from −0.1 (dB/decade) to −0.025 (dB/decade), but magnetization of the magnetic layer is reduced from 0.37 (memu/cm2) to 0.30 (memu/cm2). The signal decay is manifested in the form of reduction in output of data with passage of time. The smaller the absolute value of the signal decay, the more thermally stable the magnetic layer becomes. Specifically, the signal decay (dB/decade) is represented by the slope of a line formed by plotting output (dB) along the vertical axis, and time along the horizontal axis (common-logarithmic coordinate). As described above, when thermal stability of an AFC medium is improved through conventional techniques, output of the AFC medium is lowered.
Meanwhile, in order to improve thermal stability of a conventional AFC medium, a first magnetic layer of the AFC medium must be thickened. When the thickness of the first magnetic layer is increased, magnetic coupling between magnetic layers provided atop and beneath a non-magnetic coupling layer becomes weak.